The extent to which exercise training or long-term physical activity influences dysregulated immune function in the elderly is unclear. Preliminary evidence suggests that exercise training may improve various dysregulated immune function measures in older adults. Although such findings have the potential to be of substantial public health importance, the majority of studies have suffered from small sample sizes, inadequate measurement of physical fitness, and weak research designs. .The proposed study is designed to overcome these limitations by employing a longitudinal randomized controlled trial examining the effect of exercise training on clinically relevant immune function measures in older adults (65-80 years). Moreover, relationships between several factors known to be altered by exercise training and changes in immune function will be assessed. As such, there are two specific aims to be addressed. In Aim 1, a one-year exercise trial is proposed to determine whether moderate intensity aerobic exercise training can improve immune function in previously sedentary older adults. In Aim 2, the role played by physiological, behavioral, and psychosocial factors in the relationship between exercise training and improved immune function will be examined. Employing a randomized controlled trial, sedentary subjects (n=150) will be randomly assigned to either a one year moderate aerobic exercise training program (n = 75) or a sedentary control group (n = 75). Clinically relevant measures of immune function including the delayed-type hypersensitivity response to a battery of antigens and the antibody response to tetanus toxoid and influenza virus vaccination will be assessed before, during and after the intervention. We hypothesize that exercise training will result in improved immune responses including higher peak antibody titers and DTH responses, and sustained levels of protective antibodies. State-of-the-art analytical methods using multiple imputation and latent growth curve analyses will allow us to determine the growth and form of immune function parameters over the study period and the relative role played by physiological, behavioral, and psychosocial variables in this growth. Findings from the proposed study will help determine the role played by exercise training in immune functioning in the elderly thereby providing important empirical evidence to substantiate the prescription of exercise to combat disease and dysfunction. The public health yield of such information is likely to be substantial. In addition, the biobehavioral approach inherent in our aims will allow us to determine the complexity of the relationship between exercise and immune function.